Our objective is to resolve the regulation of nitrogenase, nitrate reductase, and nitrite reductase in relation to the ammonia utilizing enzymes glutamine synthetase, glutamate, synthase, and glutamate dehydrogenase. Nitrate and nitrite reductase and glutamine synthetase have been purified to electrophoretic homogeneity. A charcterization of effectors of these enzymes is in progress. Activity levels of these enzymes in response to different nitrogen sources have been determined. The % N of cells plays a role in increasing or decreasing the activity of the above enzymes. A large number of mutants with phenotypes indicating lesions in the above enzymes have been isolated. Comparisons between mutants and wild-type cells indicate regulation at both the genic and enzymic levels.